The present invention pertains generally to fluoropolymers and in particular to fluorinated acrylic polymers.
Acrylic polymers, whether linear or cross-linked, are excellent adhesives. They can be easily fabricated into many different components and shapes. Acrylics have exceptional optical clarity, strength, and dimensional and color stability. They have a convenient liquid-to-solid transition upon curing and can be cured by radiation.
Fluorocarbons have a much broader resistance to physical and chemical attack than acrylic polymers, but lack the advantages of acrylic polymers. This suggests that acrylics can be enhanced by the introduction into the molecules of substantial amounts of fluorine, provided such an addition does not compromise the characteristic acrylic properties. Fluorocarbons also possess a range of unusual surface chemical properties which would increase the versatility of acrylics if combined with them. For example, in the liquid precured state, a fluoroacrylic resin is expected to have a low surface tension and excellent wetting capability for difficult-to-wet fillers, such as powdered Teflon, whereas the cured fluoroacrylic can be expected to be relatively non-wetting and non-absorptive of most liquid systems, particularly those that are water based.
The problem of introducing substituted quantities of stable fluorocarbon into resins without undue compromise of the strength properties has been previously solved for epoxy systems by utilizing, as intermediates, a series of fluorinated tertiary alcohols with an aromatic nucleus surrounded by perfluorinated aliphatic groups and bearing hydroxyl functionalities derived from hexafluoroacetone as intermediates. Examples of this technique are reported in (1) J. G. O'Rear et al., Journal of Paint Technology, 43(552); p. 113, 1970; (2) J. R. Griffith et al., Synthesis, 1974(1); p. 493; and (3) D. L. Hunston et al., Ind. Eng. Chem. Prod. Res. Dev., 17(1): p. 10 (1978).
Attempts to esterify these intermediates directly by the use of acrylic acid or acrylic anhydride have not been successful. A cis-trans fluoropolyol acrylic resin has however been prepared by a method comprising reacting a cis-trans diol derived from hexafluoroacetone and propene, a fluoroaromatic diol, and epichlorohydrin in refluxing acetone with a 10% excess of aqueous sodium hydroxide to produce a fluoropolyol monomer, reacting the fluoropolyol monomer with acrylic acid anhydride, purifying the ester by percolating its ether solution through alumina, and polymerizing the ester. This method is disclosed in U.S. patent application, Ser. No. 124,203, by J. R. Griffith and J. G. O'Rear, filed on Feb. 25, 1980. The disadvantage with this acrylic resin is the limitation on the number and length of the pendant perfluoroalkyl groups.
Fluorinated acrylic polymers have been prepared from fluorinated monoacrylate monomers. One polymer of note is from phenylbis (trifluoromethyl) carbinyl acrylate, reported in J. N. Roitman and A. G. Pittman, Jr. Polym. Sci., 12, 14211-1428 (1974). This monomer was prepared by reacting phenylbis (trifluoromethyl) carbinol and acryloyl chloride in tetrahydrofuran with potassium being added to form a salt with the acid byproduct. This method developed from an earlier work reported in J. N. Roitman and A. G. Pittman, J. Polym. Sci., B, 10, 499@501, (1972) in which fluorinated monoacrylate monomers are prepared by reacting a fluorinated alcohol with acryloyl chloride in 1,1,2-trifluro-1,2,2-trichloroethane with triethylamine being added as an acid acceptor. Fluorinated diacrylate monomers cannot be prepared by the first of these methods because the solvents cannot prevent phase separation. Further, both methods use vacuum distillation to purify the product which cannot be used to purify higher-boiling diacrylate monomers due to the fast polymerization of these monomers, even under a vacuum. The addition of polymerization inhibitors would require a much higher distillation temperature which would in turn nullify the effect of the inhibitors.
Diacrylate monomers are important because their polymerization produces three-dimensional network polymers which are thermosets. In contrast, monoacrylate polymers are thermoplastic, a property detrimental to most high-temperature applications.